Crosslinking or olefin copolymers containing ketone groups using high energy radiation

ABSTRACT

OLEFIN COPOLYMERS CONTAINING KETONE GROUPS ARE CROSSLINKED BY TREATMENT WITH HIGH ENERGY RADIATION SUCH AS Y-RAYS OR ELECTRON BEAMS.

United States Patent O US. Cl. 204159.14 4 Claims ABSTRACT OF THEDISCLOSURE Olefin copolymers containing ketone groups are crosslinked bytreatment with high energy radiation such as 'y-rays or electron beams.

This is a continuation of application Ser. No. 763,980, filed Sept. 30,1968 and now abandoned.

BACKGROUND OF THE INVENTION (1) Field of the invention This inventionrelates to methods of crosslinking olefin (i.e. mono-olefin) copolymersand to products produced thereby. The invention more particularlyrelates to the production of polyethylene films for shrink packaging.

(2) Description of the prior art It is known that the properties ofpolyolefins such as polyethylene can be improved by treatment withhighenergy radiation such as X-rays from a cobalt-60 source, or electronbeams from a resonant transformer or linear accelerator. Films, fibresand molded articles of polyethylene, when treated with such radiation,tend to cross-link and this raises the melting or softening point of theresin and improves its heat resistance and other desirable propertiessuch as resistance to stress crazing. It is also possible to makeshrinkable plastic films by irradiation of the films with high-energyelectrons for shrink-packaging. Polyethylene compositions used forinsulation on electrical wiring can be irradiated to improve their heatresistance and resistance to stress crazing. These developments are thebasis for a considerable number of commercial applications, but one ofthe problems is that relatively high doses of radiation are necessary inorder to obtain the desired improvement in the properties of thepolyolefin.

Polyolefins containing ketone groups have been known for many years. Forexample, US. Pat. 2,391,218 dated Dec. 18, 1945 of R. G. R. Bacon and R.B. Richards describes the copolymerization of ethylene and carbonmonoxide to form a copolymer containing ketone groups in the backbone ofthe chain and probably also in short side chains. =Polyolefinscontaining ketone groups in side chains can be prepared bycopolymerization of ethylene or other olefins with monomers such asmethyl-vinylketone or methyl-isopropenyl-ketone. However the inclusionof ketone groups in polyolefin chains can be a major cause ofdegradation of the polymer when the polymer is subjected to ultravioletradiation (see Hartley and Guillet, Macromolecules I, p. 165 (1968)).Consequently the uses of polyolefins containing ketone groups have beenlimited and precautions are normaly taken in polymerization to excludeketone groups.

SUMMARY OF THE INVENTION Although by analogy to the eifects ofultraviolet radiation one would expect that higher energy radiationwould cause degradation of olefin copolymers containing ketone groups, Ihave found the reverse to be the case and such ice TABLE 1 [Energy andwavelength of radiation] Wavelength Energy Radiation range (cm) range(e.v.)

Ultraviolet 0. 2-4. 0X10" 3 Gamma rays. 10-10' 10 -10 Electron beams.-.10- -10" 10 -10 It will be seen that ultraviolet radiation involvesrelatively low energies (3-100 e.v.) whereas gamma raysand electronbeams are characterized by energies per quantum of about 100,000 toabout 100,000,000 e.v. All radiation having energies in this range isreferred to herein as high energy radiation.

As mentioned above it has now been found that olefin copolymerscontaining ketone groups build up in molecular weight when subjected tohigh energy radiation. Indeed, as compared to polyolefins not containingketone groups, there is substantial improvement in the radiationefiiciency where the polymer to be irradiated includes, in the back boneor in side chains or in both, from 0.01 to 50 mole percent of ketonegroups, but the range of ketone groups will normally be between about0.1 and 10 mole percent. The presence of these ketone groups, even invery low concentrations, apparently increases the efiiciency of theinteraction of the high energy radiation with the plastic to such anextent that many more cross-links are formed than in a polymer ofsimilar molecular weight but containing no ketone groups. Thus by thepresent invention it is possible to reduce the amount of high energyradiation to produce the desired improvement in properties oralternatively to achieve a greater improvement in properties of theproduct with the same dose of radiation. The cost of the irradiationprocess can be reduced because one can use faster production rates andless electrical energy to produce the desired properties.

According to the invention, an olefin copolymer is prepared with carbonmonoxide or a ketone-containing vinyl monomer. Pololefins containingbetween about 0.1 and 5 mole percent of ketone groups have all thedesirable properties of unmodified polyolefins and can in general beused in all of the applications suitable for the unmodified polyolefins.By the process of the invention, the olefin copolymer is then fabricatedinto a molded object, a fibre or a film by conventional molding orextrusion processes, and subsequently or during the molding or extrusionprocess is treated with high energy radiation from a suitable source,such as a cobalt-60 'y-cell, an electron beam generator, an atomic pile,or other source of high energy radiation. The length of time of theirradiation and the intensity are adjusted to give the desiredimprovement to the physical properties of the resin. In general thedegree of improvement depends on the radiation dose, being greater thelarger the dose of radiation to which the sample is subjected. Dosesfrom one to 50, megarads can be used to advantage in the process of thisinvention. The products of the invention may vary from tough, rubberlikeplastic materials to hard, rigid resins depending to a large extent onthe nature of the copolymer Which is subjected to the high energyradiation and the dose of radiation applied.

The invention is particularly applicable to branched polyethylenebecause there is no difiiculty in including ketone monomers therein.There is greater difiiculty with other polyolefins, but ketone monomersmay be included by block or graft copolymerization and if this issuccessful subsequent irradiation with high energy radiation willimprove the cross-linking. Although the reason for the improvementcannot be stated conclusively, hydrocarbons are relatively inert toradiation, and the high energy radiation appears to localize in theketone groups rendering them more efficient in producing free radicalswhich are available for cross-linking. To amplify the theory, when highenergy radiation such as a 'y-ray is absorbed by a polymer, asubstantial portion of the total energy is used up in producingelectrons of lower energy by Compton scattering. A relatively largenumber of such Compton electrons may be formed from a single 'y-ray.These electrons may then collide with atoms in the molecule, which atomsare thereby raised to an excited state. Chemical reactions occur fromthese excited states. The amount of chemical reaction occurring dependson various factors, such as the ease of formation of the state, thelifetime of the state, and the case of reaction from the state. It iswell known that hydrocarbon groups generally are relatively inert tohigh energy radiation. Thus, a high dose is required to obtain thedesired degree of chemical reaction. On the other hand the ketonecarbonyl appears to be particularly effective in causing chemicalreaction, particularly in the formation of free radicals which arebelieved to be the intermediates involved in the crosslinking ofpolyethylene. Thus the reaction tends to be concentrated in the regionof the carbonyl group even when these are present in relatively smallamounts. This causes an enhancement of the cross-linking reaction whenthe ketone carbonyl is present in the polymer chain.

It is known that certain compounds may be added to polyolefin films toimprove the efficiency of radiation cross-linking. For example Odian,Bernstein, Schaeffer, Friedman and Kelly (U.S. Atomic Energy CommissionNYU-2481 (1961)) showed that polyethylene can be cross-linked at lowerradiation doses when di and polyfunctional monomers are admixed with thepolymer. Useful monomers are ethylene glycol dimethacrylate,polyethylene glycol dimethacrylate, triallyl cyanurate and allylmethacrylate. Since such monomers react by a freeradical mechanism, theuse of polyolefins containing ketone groups with such additives by theprocess of the present invention should show an even greater efiiciencyfor radiation cross-linking. In general additives which improve theefl'iciency of radiation cross-linking of pure polyolefins can beexpected to be even more elfective when used with ketone-containingpolyolefins such as the ethylene-carbon monoxide copolymers.

Similarly the invention is applicable to other olefin copolymers such ascopolymers of ethylene and vinyl acetate with carbon monoxide.Preferably the copolymer includes at least 90 mole percent olefin and0.1 to 5 mole To produce a shrinkable film the olefin copolymer isextruded as a film, is exposed to high energy radiation while in themolten or partially solid state, and is then stretched. The film can bewrapped around an object such as a food product (for example, poultry ormeat) or an electrical connection, and its temperature is then raiseduntil the cross-linking causes it to shrink snugly around the object.Film produced according to the invention can be free of toxic orextractable additives which must be avoided in food processing. This isin contrast to polyolefin films which have been cross-linked by use ofcuratives, such as peroxy or azo compounds, and which therefore containresidues derived from such curatives.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The process and products of theinvention are illustrated in but not restricted by the followingexamples.

Example 1 A series of copolymers of ethylene and carbon monoxide wereprepared by a high pressure continuous polymerization process of thekind described in British Pat. 471,590 dated Sept. 6, 1937 to Fawcett,Gibson, Perrin, Paton, Williams and 1.01., including small amounts ofcarbon monoxide in the ethylene fed to the reactor. By controlling thepressure of the reactor the melt index of the sample was maintained at2. By this procedure, samples containing 0.1, 0.5 and 1.0 mole percentof carbon monoxide, as determined by analysis of reactants and products,were prepared. For comparison, a sample containing no carbon monoxidewas also prepared. The polymer samples were extruded and chopped.

into pellets. These polymers were characterized by having a melt indexof 2, an approximate viscosity molecular weight of 22,000 and a meltingrange from 90 to 120 C. Those containing carbon monoxide exhibited astrong absorption band in the infra-red at a wavelength of 5.8 mg and abroad absorption in the ultraviolet with a maximum around 290 m Theintensity of the absorption was proportional to the amount of carbonmonoxide in the polymer. These bands are characteristic of ketonecarbonyl groups.

The pellets were dried, extracted with hexane to remove any anti-oxidantthat might be present, and sealed in glass tubes under vacuum for theirradiation experiments. The sealed tubes were irradiated for varioustimes in a 20,000 Curie cobalt- 'y-source to provide doses ranging from0.5 to 2 megarads. Viscosity measurements were then made in Tetralin atC. The molecular weights were calculated from the viscosity measurementsaccording to the following relation:

where [1;] is the intrinsic viscosity and M is the molecular weight. Theresults of a series of experiments are shown in Table II below.

TABLE II T Intrinsic viscosity in Tetralin at 80 C. Viscosity, averagemolecular wt.

nne Dose (megarads) (minutes) 0% CO 0.1% CO 0.5% CO 1.0% CO 0% CO 0.1%CO 0.5% CO 1.0% CO percent of ketone groups. The ketone-containingcopolymers can be made by any known method, for example by the methodsdisclosed in the following patent specifications: U.S. 2,495,286; U,S.2,519,791; U.S. 2,557,256; U.S. 2,391,920 and U.S. 2,405,950.

It is well known that the rate of formation of cross-links for polymershaving the same starting molecular weight is proportional to the slopeof the line obtained by plotting the reciprocal of the molecular weightas. a. function TABLE III Concentration of carbon monoxide in copolymer(mole percent):

Relative rate of formation of cross-links From these results it can beseen that the inclusion of only 1.0 mole percent carbon monoxide in theethylene copolymer increases the rate of formation of cross-links by afactor of about 3 and even 0.1 mole percent carbon monoxide causes anincrease of about 60% in the radiation efiiciency for the process.Polyethylene containing amounts of carbon monoxide of 1 mole percent orless have physical properties essentially indistinguishable from thoseof pure polyethylene. Consequently these polymers can be used in allapplications for which high pressure polyethylene is now used.

Example 2 Viscosity measurements cannot be used to estimate theefiiciency of cross-linking after the polymer becomes insoluble becauseof cross-linking. However, estimates of the efliciency of cross-linkingon insoluble polymers can be made by measuring the relative amounts ofgel and sol produced in the irradiation. In general, the higher theamount of gel fraction and the lower the amount of soluble fraction thehigher is the amount of cross-linking. Films were pressed fromunextracted pellets prepared (but not irradiated) as described inExample 1 by molding the pellets in a Carver press at a temperature of140 C. and a pressure of 20,000 lbs. per square inch. The films were 5mils thick and were placed in glass tubes and irradiated in the presenceof air at doses exceeding the gel dose. The sol fraction was obtained byextracting the film after irradiation with toluene for 48 hours. Theresults are given in Table IV below.

TAB LE IV [Gel fractions for irradiated polymers] Mole percent 00Radiation dose 0 1. 0 5. 3

7.8 megarads 12% 18% 63% 13.5 megarads 12% 56% 92% Example 3 A copolymerof ethylene was prepared by injecting methyl-vinyl-ketone into theethylene feed in a continuous high pressure reactor. By adjustment ofthe concentration of methyl-vinyl-ketone a polymer was preparedcontaining 1 mole percent of methyl-vinyl-ketone (i.e. 1 mole percent ofketone groups). A film was prepared by compression molding of thepellets, as in Example 2, and irradiated in a cobalt-60 'y-cell to adose of megarads. The gel fraction of this polymer was 69% compared to12% for a polymer of similar molecular weight containing no ketonegroups.

Example 4 mined by extraction in hot toluene. The results are given inTable V below.

TABLE V CO content (mole percent): Gel fraction (percent) 0 nu 13 0.1 210.5 35 1. 0 65 Example 6 A film was pressed from a 1 mole percentethylene carbon monoxide copolymer using a Carver Press at a temperatureof 150 C. and 15,000 p.s.i. The thickness of the resulting film wasapproximately 2.5 mils. The film was then irradiated in the cobalt-60source to a dose of 3.5 megarads. It was then stretched at a temperatureof 50 C. to approximately twice its original length. A portion of thefilm was wrapped around a 12 gauge copper wire and then warmed with ahot air gun to approximately C. The film shrank to form a tight bondwith the metallic conductor. Biaxial stretching of the irradiated filmcaused the film to shrink in both directions.

Example 7 A graft copolymer of methyl-vinyl-ketone on polypropylene wasprepared by the following procedure: Ten grams of finely-dividedpolypropylene powder was irradiated under vacuum in a Pyrex tube in thecobalt-60 'y-source to a dose of 2 megarads. The tube was then attachedto a vacuum line, and 10 grams of pure methyl-vinyl-ketone (MVK) weredistilled into the tube under vacuum. The tube was allowed to remainovernight at room temperature (25 C.), then the unpolymerized MVK wasremoved by vacuum distillation. The polymer powder was then extractedwith acetone in a Soxhlet extractor to remove any ungrafted poly MVK.The resulting powder was pressed into a thin film in a Carver press atC.

Based on the intensity of the infra-red band at 5.7 m it was estimatedthat the polymer contained 8.5 mole percent grafted MVK. Samples of thefilm were irradiated in vacuum at doses of 5 and 10 megarads along witha. control sample of pure polypropylene film. The gel fractions werethen determined by extraction in hot toluene. The values obtained arelisted in Table VI below.

What I claim as my invention is:

1. The process of cross-linking an olefin copolymer containing at leastabout 0.1 mole percent and no more than about 5 mole percent keto groupsand selected from the group consisting of a copolymer consistingessentially of a lower mono-olefin with carbon monoxide and a copolymerconsisting essentially of a lower mono-olefin with a ketogroup-containing vinyl monomer, which comprises irradiating thecopolymer with radiation having an energy per quantum of 100,000 to100,000,000 electron volts, the dose of irradiation being at least 0.5megarads.

2. The process of producing a cross-linked polymer which comprisescopolymerizing monomers consisting essentially of a lower mono-olefinand carbon monoxide or a keto group-containing vinyl monomer whereby theresulting copolymer contains at least about 0.1 mole percent and no morethan about 5 mole percent keto groups, and irradiating the copolymerwith radiation having an energy per quantum of 100,000 to 100,000,000electron volts, the dose of irradiation being at least 0.5 megarads.

3. The process of claim 2 wherein the copolymer contains at least 90mole percent olefin and about 0.1 to 5 mole percent ketone groups, andirradiating the copoly- 7 8 met with radiation having an energy perquantum of FOREIGN PATENTS 100,000 t0 100,000,000 electron V0118. 527219 7 195 Canada 2 o CO 4. The process of claim 3 wherein the copolymeris an ethylene-carbon monoxide copolymer. MURRAY TILLMAN, PrimaryExaminer 5 References Cited R. B. TURER, Assistant Examiner UNITEDSTATES PATENTS 1 3,162,623 12/1964 Cairns et a1 26063 R 3,530,109 9/1970Fenton 260-63 CO 10 260-63 UO, 63 UY 3,057,791 10/1962 Anderson204159.14

2,921,006 1/1960 Schmitz et al. 204-159.15

